Pseudogap and the Fermi surface in the t-J model

We calculate spectral functions within the t-J model as relevant to cuprates in the regime from low to optimum doping. On the basis of equations of motion for projected operators an effective spin-fermion coupling is derived. The self-energy due to short-wavelength transverse spin fluctuations is shown to lead to a modified self-consistent Born approximation, which can explain strong asymmetry between hole and electron quasiparticles. The coupling to long-wavelength longitudinal spin fluctuations governs the low-frequency behavior and results in a pseudogap behavior, which at low doping effectively truncates the Fermi surface.     

Fermi surfaces and quasi-particle band dispersions of the iron pnictides superconductor KFe2As2 observed by angle-resolved photoemission spectroscopy

We have performed an angle-resolved photoemission study of the iron pnictide superconductor KFe₂As₂ with . Most of the observed Fermi surfaces show almost two-dimensional shapes, while one of the quasi-particle bands near the Fermi level has a strong dispersion along the k_z direction, consistent with the result of a band-structure calculation. However, hole Fermi surfaces α and ζ are smaller than those predicted by the calculation while other Fermi surfaces are larger. These observations are consistent with the result of a de Haas-van Alphen study and a theoretical prediction on inter-band scattering, possibly indicating many body effects on the electronic structure.     

The first-principles studying LaOMnSe: A possible parent compound of superconductor

By the first-principles calculations, we studied the structure, electronic and magnetic properties of LaOMnSe. The band structure and Fermi surface of LaOMnSe are very similar to those of LaOFeAs, where there are three hole-like Fermi surfaces around Γ-point and two electron-like Fermi surfaces around M-point. The hole-like Fermi surfaces will strongly overlap with electron-like Fermi surfaces if they are shifted by the q vector (π,π,0). Such Fermi surfaces nesting will induce magnetic instability and spin density wave (SDW), which is similar to LaOFeAs. Because of so much similarity to LaOFeAs, LaOMnSe is expected to become superconductor with electron or hole doping.     

Holographic Fermi arcs and a d-wave gap

We study fermion correlators in a holographic superfluid with a d-wave (spin two) order parameter. We find that, with a suitable bulk Majorana coupling, the Fermi surface is anisotropically gapped. At low temperatures the gap shrinks to four nodal points. At high temperatures the Fermi surface is partially gapped generating four Fermi arcs.     

Breakup of the Fermi surface near the mott transition in low-dimensional systems

We investigate the Mott transition in weakly coupled one-dimensional (1D) fermionic chains. Using a generalization of dynamical mean field theory, we show that the Mott gap is suppressed at some critical hopping t{ perpendicular}{c2}. The transition from the 1D insulator to a 2D metal proceeds through an intermediate phase where the Fermi surface is broken into electron and hole pockets. The quasiparticle spectral weight is strongly anisotropic along the Fermi surface, both in the intermediate and metallic phases. We argue that such pockets would look like "arcs" in photoemission experiments.     

Electronic structure reconstruction of CaFe2As2 in the spin density wave state

The electronic structure of CaFe₂As₂, a parent compound of iron-based superconductors, is studied with high-resolution angle-resolved photoemission spectroscopy. The electronic structure of CaFe₂As₂ in the paramagnetic state is consistent with that of density-functional theory calculations. We show that the electronic structure of this compound is significantly reconstructed when entering the spin density wave state. We could resolve two hole-like pockets and four electron-like pockets around the (0, 0) point, and one electron-like pocket surrounded with a pair of electron- and hole-like pockets around the (π, π) point in the spin density wave state. Therefore, the complicated Fermi surface topology and electronic structure near Fermi surface of CaFe₂As₂ illustrate that there exists unconventional electronic reconstruction in the spin density wave state, which cannot be explained by the band folding and Fermi surface nesting pictures.     

Effect of hole doping on the electronic structure and the Fermi surface in the Hubbard model within norm-conserving cluster pertubation theory

The concentration dependences of the band structure, spectral weight, density of states, and Fermi surface in the paramagnetic state are studied in the Hubbard model within cluster pertubation theory with 2 × 2 clusters. Representation of the Hubbard X operators makes it possible to control conservation of the spectral weight in constructing cluster perturbation theory. The calculated value of the ground-state energy is in good agreement with the results obtained using nonperturbative methods such as the quantum Monte Carlo method, exact diagonalization of a 4 × 4 cluster, and the variational Monte Carlo method. It is shown that in the case of hole doping, the states in the band gap (in-gap states) lie near the top of the lower Hubbard band for large values of U and near the bottom of the upper band for small U. The concentration dependence of the Fermi surface strongly depends on hopping to second (t′) and third (t″) neighbors. For parameter values typical of HTSC cuprates, the existence of three concentration regions with different Fermi surfaces is demonstrated. It is shown that broadening of the spectral electron density with an energy resolution typical of contemporary ARPES leads to a pattern of arcs with a length depending on the concentration. Only an order-of-magnitude decrease in the linewidth makes it possible to obtain the true Fermi surface from the spectral density. The kinks associated with strong electron correlations are detected in the dispersion relation below the Fermi level.     

Spectral properties of incommensurate charge-density wave systems

The concept of frustrated phase separation is applied to investigate its consequences for the electronic structure of the high T _c cuprates. The resulting incommensurate charge density wave (CDW) scattering is most effective in creating local gaps in k-space when the scattering vector connects states with equal energy. Starting from an open Fermi surface we find that the resulting CDW is oriented along the (10)- and (or) (01)-direction which allows for a purely one-dimensional or a two-dimensional “eggbox type” charge modulation. In both cases the van Hove singularities are substantially enhanced, and the spectral weight of Fermi surface states near the M-points, tends to be suppressed. Remarkably, a leading edge gap arises near these points, which, in the eggbox case, leaves finite arcs of the Fermi surface gapless. We discuss our results with repect to possible consequences for photoemission experiments. Received 14 June 1999     

Reconstructed Fermi surface of underdoped Bi2Sr2CaCu2O(8+δ) cuprate superconductors

The Fermi surface topologies of underdoped samples of the high-T(c) superconductor Bi2Sr2CaCu2O(8+δ) have been measured with angle resolved photoemission. By examining thermally excited states above the Fermi level, we show that the observed Fermi surfaces in the pseudogap phase are actually components of fully enclosed hole pockets. The spectral weight of these pockets is vanishingly small at the magnetic zone boundary, creating the illusion of Fermi "arcs." The area of the pockets as measured in this study is consistent with the doping level, and hence carrier density, of the samples measured. Furthermore, the shape and area of the pockets is well reproduced by phenomenological models of the pseudogap phase as a spin liquid.     

Photoemission studies on electron doped cuprate Pr0.85LaCe0.15CuO4: Revisiting the chemical pressure effect

We performed angle resolved photoemission spectroscopy (ARPES) experiments on electron doped cuprates Pr_0.85LaCe_0.15CuO₄ (PLCCO) and Nd_1.85Ce_0.15CuO₄ (NCCO). Critical temperatures (T_c) of PLCCO and NCCO are similar but PLCCO has weaker Fermi surface curvature than NCCO. As the ionic radius of Pr and La is larger than that of Nd, this result is inconsistent with the earlier view that chemical pressure determines the Fermi surface curvature. On the other hand, anti-ferromagnetic (AFM) band renormalization effect in PLCCO is larger than that in NCCO, which implies AFM is stronger in PLCCO. This is consistent not only with the view that AFM is correlated with t′/t but also with recent inelastic neutron scattering results. Therefore, we suggest that the chemical pressure effect is not the only factor that determines the Fermi surface topology.     

Near EF electronic structure of heavily boron-doped superconducting diamond

We have performed soft X-ray angle-resolved photoemission spectroscopy (SXARPES) of a heavily boron-doped superconducting diamond film (T_c=7.2 K) in order to study the electronic structure near the Fermi level (E_F). Careful determination of measured momentum space that across Γ point in the Brillouin zone (BZ) and increase of an energy resolution provide further spectroscopic evidence that E_F is located at the highly dispersive diamond-like bands, indicating that holes at the top of the diamond-like valence band play an essential role for the conducting properties of the heavily boron-doped superconducting diamond for this boron-doping region (effective carrier concentration of 1.6%). The SXARPES intensities at E_F were also mapped out over BZ to obtain experimental Fermi surface sheets and compared with calculations.     

Fermi surface and metallic properties of graphite at high pressures

A potential of superconductivity of pure graphite has been theoretically examined. At normal pressure, the carrier concentration is too low to exhibit superconductivity. On applying pressure, the band dispersion along the c-axis is significantly enhanced, resulting in an increase in the carrier concentration; 10²⁰ cm^-3 at p=30 GPa. This is favorable to observe superconductivity. Accurate Fermi surfaces are illustrated: a new Fermi surface appears around K point at p=25 GPa.     

Calculation of the Fermi velocity in tungsten

The anisotropy of the Fermi velocity of tungsten has been calculated for different parts of the Fermi surface. The calculation was carried out by using the LMTO method in which the potential was constructed according to Mattheiss' rule with overlapping, relativistic atomic charge densities. In accordance with the experiments the results show that both magnitude and the anisotropy of the Fermi velocity are highest for the hole octahedron surface. For the other parts of the Fermi surface the value of the Fermi velocity is smaller, and it does not exceed a 50% change as a function of k.     

First principles simulations of energy and polarization dependent angle-resolved photoemission spectra of Bi2212

We discuss first-principles simulations of angle-resolved photoemission (ARPES) intensity in Bi2212 where the photoexcitation process is modeled realistically by taking into account the full crystal wavefunctions of the initial and final states in the presence of the surface. Some recent results aimed at understanding the effects of the energy and polarization dependencies of the ARPES matrix element are presented. The nature of the Fermi surface (FS) maps obtained via ARPES by holding the initial state energy fixed at the Fermi energy (E_F) is clarified. The theoretically predicted FS map at 21 eV photon energy displays a remarkable level of agreement with the corresponding ARPES spectrum taken over a large area of the (k_x,k_y) plane. Our analysis shows how the ARPES matrix element can help disentangle closely spaced energy levels and FS sheets and highlight different aspects of the electronic spectrum in complex materials under various experimental conditions.     

Effect of canted antiferromagnetic order on the electronic structure in the <Emphasis Type="Italic">t–J</Emphasis>* model within the cluster perturbation theory

The electronic structure in the two-dimensional t–J* model with canted antiferromagnetic order in an external magnetic field has been calculated within the cluster perturbation theory. In zero external field, the evolution of the Fermi surface with n-type doping has been obtained in good agreement with experimental data on cuprate superconductors. It has been shown that the inclusion of short-range correlations can result in a nonmonotonic dependence of the spectral weight distribution at the Fermi level on the external magnetic field. In contrast to the case of electron doping, such changes in the case of hole doping can be expected at experimentally achievable fields.     

Fermi surface and helical spin ordering in Eu metal

The band structure and the Fermi surface of the b.c.c. Eu metal have been calculated by the non-relativistic KKR method. The obtained Fermi surface is grossly similar to that of Andersen and Loucks. We should like, however, to point out essential differences. Our electron surface around H is shaped like a sphere with eight low mounds in eight directions near the HP-axes (called “bumpy sphere”). Our hole surface around P is shaped like a “rounded-off cube”, as was obtained by Andersen and Loucks, but the cross-section perpendicular to the HP-axis for the tetrahedrally located wing (or island) is shaped like a “truncated-triangle”, not like an ellipse. The helical spin ordering at low temperatures can be understood on the basis of the hole Fermi surface. The electron surface around H does not seem to concern the nesting. Some other phenomena are also considered briefly.     

Anisotropic scattering rates and antiferromagnetic precursor effects in the t-t'-U Hubbard model

We have investigated the evolution of the electronic properties of the t-t'-U Hubbard model with hole doping and temperature. Due to the shape of the Fermi surface, scattering from short wavelength spin fluctuations leads to strongly anisotropic quasi-particle scattering rates at low temperatures near half-filling. As a consequence, significant variations with momenta near the Fermi surface emerge for the spectral functions and the corresponding ARPES signals. At low doping the inverse lifetime of quasiparticles on the Fermi surface is of order varying linearly in temperature from energies of order t down to a very low energy scale set by the spin fluctuation frequency while at intermediate doping a sub-linear T-dependence is observed. This behavior is possibly relevant for the interpretation of photoemission spectra in cuprate superconductors at different hole doping levels. Received 31 July 2000     

Theoretical study on the superfluid density of the superconducting MNCl (M=Hf, Zr)

We consider the possibility of spin fluctuation mediated d+id′‐wave pairing for the layered nitride superconductor MMCl. Using the superconducting gap obtained within the fluctuation exchange method for the two band model, we calculate the superfluid density as a function of temperature for several doping concentrations. Reflecting the fact that the anisotropy of the d+id′‐wave gap on the Fermi surface is enhanced with the increase of the doping concentration, the overall doping dependence of the superfluid density is in qualitative agreement with the experiment.     

Numerical evidence of Luttinger liquid to Fermi liquid transformation in lithium doped trans-polyacetylene chain

We report on the first direct numerical evidence of doping-induced transformation of Tomonaga-Luttinger liquid to Fermi liquid in quasi-one-dimensional lithium doped trans-polyacetylene chain. Using density functional theoretical calculation, an analysis of density of states near the Fermi energy reveals a power-law scaling factor of Tomonaga-Luttinger liquid at low dopant concentration in the metallic regime. As soon as the doping level reaches 0.0763e/C, normal power-law scaling factor of Fermi liquid has been realized as a special case of Luttinger liquid in one dimension. The variation of density-density correlation is consistent with the present theoretical prediction.     

The influence of out-of-plane disorder on the formation of pseudogap and Fermi arc in Bi2Sr2−xRxCuOy (R=La and Eu)

We found that the length of the Fermi arc decreases with increasing out-of-plane disorder by performing angle resolved photoemission spectroscopy (ARPES) measurements in the superconducting state of optimally doped R=La and Eu samples of Bi₂Sr_2−xR_xCuO_y. Since out-of-plane disorder stabilizes the antinodal pseudogap as was shown in our previous study of the normal state, the present results indicate that this antinodal pseudogap persists into the superconducting state and decreases the Fermi arc length. We think that the shrinkage of the Fermi arc reduces the superfluid density, which explains the large suppression of the superconducting transition temperature when out-of-plane disorder is increased.